Method of manufacturing a composite material with lamellar interphase between reinforcing fibers and matrix, and material obtained

ABSTRACT

The interphase is formed by nanometric scale sequencing of a plurality of different constituents including at least a first constituent that intrinsically presents a lamellar microtexture, and at least a second constituent that is suitable for protecting the first against oxidation. A plurality of elementary layers of a first constituent of lamellar microtexture, e.g. selected from pryolytic carbon, boron nitride, and BC 3  are formed in alternation with one or more elementary layers of a second constituent having a function of providing protection against oxidation and selected, for example, from SiC, Si 3  N 4 , SiB 4 , SiB 6 , and a codeposit of the elements Si, B, and C. The elementary layers of the interphase are preferably less than 10 nanometers thick and they are formed by chemical vapor infiltration or deposition in pulsed form.

FIELD OF THE INVENTION

The present invention relates to manufacturing composite materialscomprising fiber reinforcement densified by means of a matrix, andhaving a lamellar interphase between the reinforcing fibers and thematrix.

A particular field of application of the invention is that ofthermostructural composite materials. Such materials are characterizedby mechanical properties that make them suitable for constitutingstructural elements, and by the ability to retain their mechanicalproperties up to high temperatures. Thermostructural composite materialsare used, in particular, for making parts of engines or of reactors, orfor making structural elements of space vehicles which are exposed tosevere heating.

BACKGROUND OF THE INVENTION

Examples of thermostructural composite materials include carbon/carbon(C/C) composites comprising carbon fiber reinforcement and a carbonmatrix, and ceramic matrix composites (CMC) comprising refractory(carbon or ceramic) fiber reinforcement and a ceramic matrix. CommonCMCs are C/SiC composites (carbon fiber reinforcement and a siliconcarbide matrix) and SiC/SiC composites (reinforcing fibers based onsilicon carbide, and a silicon carbide matrix).

Composite materials in which the reinforcement is constituted by longfibers are known to possess greater toughness and greater mechanicalstrength than the corresponding monolithic materials.

In the case of thermostructural composites, it is also known thatgreater toughness is obtained by interposing an interphase between thefibers and the matrix, the interphase serving to transfer load from thematrix to the fibers while simultaneously deflecting cracks that appearin the matrix when the material is subjected to mechanical stress,thereby ensuring that the cracks do not propagate to the fibers, andsimultaneously relieving residual stresses at the bottoms of the cracks.

To achieve these objects, the Applicants' document EP-A-0 172 082proposes forming an interphase on the reinforcing fibers prior todensification of the matrix, the microtexture of the interphase beinglamellar. That is achieved by forming on the fibers a layer of pyrolyticcarbon (PyC) of the rough laminar type, or a layer of boron nitride (BN)obtained by chemical vapor infiltration or deposition. The stacks ofsheets of atoms of PyC or of BN impart the lamellar microtexture to theinterphase. In the resulting final material, when a crack reaches theinterphase after propagating through the matrix, its mode of propagationis modified so that the crack is deflected parallel to the sheets ofatoms in the interphase, i.e. parallel to the fiber, thereby protectingthe fiber. In addition, because of its elastic nature in shear, the PyCor BN lamellar interphase serves to relieve the stresses at the bottomof the crack. By preserving the fibers in the cracked material, thematerial conserves its integrity and its mechanical properties, andconsequently it presents much greater toughness than the same matrixmaterial when it is monolithic.

It is well known that the microtexture of a PyC obtained by chemicalvapor infiltration or deposition depends on infiltration or depositionconditions, and in particular on temperature and pressure. Thus,depending on conditions, it is possible to obtain PyCs that are highlyanisotropic (lamellar microtexture), such as PyC of the rough laminartype, or PyCs that are not very anisotropic, (non-lamellarmicrotexture), such as PyC of the smooth laminar type. Unfortunately,during deposition of a PyC interphase whose thickness is typicallygreater than one hundred nanometers, it has been observed that themicrotexture of the PyC can vary within the interphase, going from therough laminar type to the smooth laminar type, and that this can happenwithout deposition conditions changing. Such uncontrolled variationmeans that the interphase no longer has optimum microtexture, with themain consequence of the mechanical properties of the composite materialbeing less good than could have been expected from the reinforcingcapacity of the fibers.

The person skilled in the art also knows that PyC interphase compositesare poor at withstanding prolonged exposure to an oxidizing atmosphereat high temperature and under mechanical stress. This weaknessconstitutes a major limitation on the use of SiC matrix composites witha PyC interphase, and it is due to the property of the PyC interphasewhereby it oxidizes as soon as the temperature reaches 450° C. to 500°C., forming volatile oxides (CO₂ and/or CO, depending on temperature),thereby causing an annular pore to be formed around each fiber. Suchoxidation is made easier by the cracking of the matrix under mechanicalstress and, other things remaining equal, it becomes easier withincreasing number of active sites, i.e. with increasing imperfection ofthe structure of the PyC microtexture.

Oxidation of the interphase can have two types of consequence: it candestroy fiber-matrix coupling (load is no longer transferred betweenthem), or it can "unite" the fibers to the matrix with the compositematerial then becoming brittle (catastrophic propagation of matrixcracks to the fibers), with this depending on the nature of the matrix,the thickness of the interphase, and conditions of use. In practice,deep oxidation of the PyC interphase frequently leads to total loss ofthe mechanical properties of the composite material.

The use of a BN interphase improves to some extent the behavior ofcomposite materials in an oxidizing environment compared with the use ofa PyC interphase. However, a BN interphase suffers from the samedrawback as a PyC interphase, i.e. it is impossible to controlaccurately the microtexture of the interphase throughout the thicknessthereof. As a general rule, BN interphases formed by chemical vaporinfiltration or deposition do not have a suitable lamellar texture forenabling them to perform the looked-for functions effectively.

To avoid using materials of lamellar microtexture, such as PyC and BN,with the drawbacks that they entail, in particular insufficientresistance to oxidation at high temperatures, it is proposed in documentFR-A-2 673 937 to make an interphase that is not oxidizable, being madeup of a plurality of layers that impart a generally laminated structurethereto by mechanical means. The layers making up the interphase aremade of oxide type ceramic (e.g. alumina or zirconia) or non-oxide typeceramic (e.g. silicon carbide or silicon nitride). In order to conserveweak bonding between the layers, thereby giving the overall interphaseits laminated texture, the layers are made of different ceramics duringdistinct chemical vapor deposition steps. Proposals have also been madeto form ceramic layers that have different morphologies, with chemicalvapor deposition conditions being varied from one layer to another.Proposals have also been made to inhibit chemical bonding between layersof the same ceramic by doping the layers with impurities or by modifyingtheir surface states.

The above solutions require a plurality of chemical vapor depositions tobe performed, either under different conditions or else withintermediate steps. They are therefore lengthy and expensive toimplement. In addition, the number of layers making up the interphasecannot be too great, since that requires a corresponding number ofchemical vapor deposition operations. As a result the laminated natureremains limited (five to ten layers) and the thickness of the layer(several tens of nanometers) is much greater than the distance betweensheets of atoms (about 0.33 nanometers) in PyC or BN interphases oflamellar microtexture.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method enabling a compositematerial to be made with a lamellar interphase between reinforcingfibers and a matrix, and having improved thermomechanical properties inan oxidizing medium.

A particular object of the invention is to provide a method enablinginterphases to be formed of controlled lamellar microtexture capable ofwithstanding oxidation without requiring lengthy and expensivedeposition operations.

These objects are achieved by a method of the type comprising makingfiber reinforcement in which the fibers are provided with a lamellarinterphase coating made up of a plurality of layers, and densificationby means of a matrix for the reinforcing fibers provided with thelamellar interphase, in which method, according to the invention, thelamellar interphase is formed by a nanometric-scale sequence of aplurality of different constituents of which at least a firstconstituent intrinsically presents a lamellar microtexture and at leasta second constituent is suitable for protecting the first againstoxidation.

Herein, "nanometric scale sequence of a plurality of constituents" meanssuccessively forming elementary layers each of nanometer orderthickness, i.e. layers that are each preferably less than 10 nanometersthick.

One or more nanometric layers of a first constituent having lamellarmicrotexture, e.g. PyC, BN, or BC₃, can be formed in alternation withone or more nanometric layers of a second constituent having a functionof providing protection against oxidation. The second constituent ispreferably a refractory material having healing properties, eitherintrinsically or via an oxidation product. At the temperatures at whichthe composite material is used, this healing function is provided bytaking up a semi-liquid state enabling any pores that may appear in theinterphase to be plugged and "coating" the constituent having lamellarmicrotexture. Materials suitable for this purpose are those capable ofgiving rise to glasses, in particular materials based on silica and/oron boron. Mention may be made in particular of silicon carbide (SIC) orsilicon nitride (Si₃ N₄) which give silica on being oxidized, and ofsilicon borides (SiB₄, SiB₆) or compounds taken from the triplet SiBC(co-deposition of the elements Si, B, and C) that give borosilicateglass on oxidation. The choice of material depends in particular on theconditions in which the composite material is to be used, so as toensure that the glass takes up a semi-liquid state at the temperature ofuse.

A feature of the invention thus consists in associating, within theinterphase, at least a first constituent of lamellar microtexture, andat least a second constituent having a function of providing protectionagainst oxidation. An interphase is thus obtained having a lamellarmicrotexture with integrated protection against oxidation.

Another feature of the invention consists in implementing the interphaseby nanometric sequencing. This is preferably performed by chemical vaporinfiltration or deposition in a chamber within which a plurality ofsuccessive cycles are performed each comprising injecting a reaction gasand maintaining it within the chamber for a first time interval ofpredetermined duration so as to form an elementary interphase layer ofcontrolled nanometer-order thickness, followed by evacuating the gaseousreaction products during a second time interval, with the cycles beingperformed consecutively in the chamber until the interphase has reachedthe desired thickness.

The microtexture and the thickness of each layer can then be controlledaccurately by precise conditions determined by the chemical vaporinfiltration or deposition and by its duration, during each cycle. Thisavoids any undesirable change of the microstructure, as has beenobserved in the prior art when the interphase is formed in a singlechemical vapor infiltration or deposition operation.

Advantageously, the elementary layers of the interphase are formedduring consecutive cycles, while the reinforcing fibers remain in theenclosure within which the chemical vapor infiltration or depositionoperations are performed. Each first portion of a cycle, during which areaction gas is admitted into and maintained within the chamber until ananometric elementary layer has been obtained, lasts for a duration thatmay be limited to a few seconds or a few tens of seconds. Each secondportion of a cycle, during which the gaseous reaction products areevacuated from the chamber, e.g. by pumping and by sweeping with aninert gas, has a duration that normally does not exceed one or a fewseconds. Because the cycles follow one another consecutively, andadvantageously without interruption, and because the duration of eachcycle is short, the total time required for forming the interphase isrelatively short, even when several tens of cycles are necessary.

Contrary to the teaching of document FR-A-2 673 937, no specialprecautions need to be taken to avoid bonding between the elementarylayers that are formed in succession. The lamellar nature of theinterphase is provided by the first constituent thereof, and not by thelamination due to the formation of mutually non-bonded layers.

As already mentioned, one or more consecutive layers of the firstconstituent having lamellar microtexture may alternate with one or moreconsecutive layers of the second constituent. The layers of the firstconstituent and the layers of the second constituent may be ofthicknesses that are equal or unequal. The thicknesses may be constantthroughout the interphase or they may vary, with thickness variationbeing controlled by varying parameters of the chemical vaporinfiltration or deposition (partial pressures of the components of thereaction gas in the reaction chamber, durations of the first portions ofthe cycles, . . . ).

By varying the thicknesses of the layers of the first and/or of thesecond constituent and/or by varying the ratio between the number oflayers of the first constituent and the number of layers of the secondconstituent, it is possible to vary the proportion of at least one ofthe constituents across the thickness of the interphase so as to obtaina desired composition gradient.

In a particular implementation of the invention, one of the twoconstituents can be obtained by modifying the reaction gas that providesthe other constituent, e.g. by adding a component to said gas whichdisturbs deposition and which imparts different characteristics thereto.

Another possibility consists in performing cycles in which the durationof the first portion (admitting and maintaining the reaction gas) takesdifferent values. Lengthening the duration during which the gas admittedinto the enclosure is maintained therein without external communicationhas the effect of depleting the gas which, beyond some limit, can causethe nature of the deposition to be modified. For example, a reaction gasthat causes silicon carbide to be deposited may, after some length oftime, give rise to said gas being depleted, whereupon both siliconcarbide and carbon will be codeposited. The passage from one interphaseconstituent to the other thus takes place merely by varying the durationof deposition within a cycle.

The lamellar interphase may be formed on the fibers of the fiberreinforcement at any stage in the manufacture of the reinforcement fromfiber roving to a made-up multidirectional fiber preform having theshape of a part of composite material that is to be made, and includingvarious intermediate stages, e.g. a fabric obtained by weaving fiberroving. Nevertheless, it is preferable for the interphase to be madedirectly on the preform, i.e. the last stage in preparation of the fiberreinforcement.

DESCRIPTION OF THE DRAWINGS

The invention will be better understood on reading the followingdescription of examples given by way of non-limiting indication.

Reference is made to the accompanying drawings, in which:

FIG. 1 is a highly diagrammatic overall view of an installation suitablefor implementing the method of the invention; and

FIG. 2 shows how pressure varies as a function of time within thechemical vapor infiltration chamber of the FIG. 1 installation whileimplementing the method of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

An installation for implementing the method of the invention is shown inFIG. 1. The installation is of the type commonly used for performingchemical vapor infiltration operations. In conventional manner itcomprises a graphite core 10 defining a reaction chamber 12. The core 10is surrounded by a metal inductor 14 with thermal insulation beinginterposed therebetween. The assembly comprising the core 10 and theinductor 14 may be housed within a sealed enclosure, e.g. as describedin document WO-A-87/04733.

A fiber substrate 18 whose fibers are to be coated in an interphase oflamellar microtexture is placed within the chamber 12. The state of thesubstrate 18 may be that of fiber roving, threads, cloth, or some othertwo-dimensional structure (sheets of unidirectional threads or cables,layers of felt, . . . ), or it may be a three-dimensional structureconstituting a preform for a part of composite material that is to bemade in accordance with the invention. The interphase is formed on thefibers of the substrate 18 by sequentially depositing nanometricelementary layers of various different constituents. Each elementarylayer is formed by admitting a reaction gas into the chamber 12, therebygiving rise to the looked-for deposit under predetermined conditions ofpartial pressure for the, or each, constituent of the gas in thechamber, and of temperature within the chamber, by decomposition of thegas or by reaction of its constituents on coming into contact with thefibers of the substrate 18.

Gases suitable for forming deposits of the desired kind are admitted tothe top of the chamber 12 from gas sources 20a, 20b, 20c, . . . viarespective injection valves 22a, 22b, 22c, . . . . In some cases, theconstituents of the gas come from different sources and are mixedtogether on being admitted into the chamber 12.

The number and the kinds of gas sources depend on the constituentsselected for the elementary layers making up the interphase. By way ofnon-limiting example, the following sources may be provided:

a source of an alkane, in particular propane, or a mixture of alkanes,which, on decomposing, can give rise to a deposit of PyC;

a source of methyltricholorosilane (MTS) CH₃ SiCl₃ and a source ofhydrogen (H₂), MTS giving rise to a deposit of SiC in the presence of H₂acting as a catalyst;

a source of ammonia (NH₃) and a source of boron trifluoride (BF₃) which,when admitted separately into the chamber, react therein to give rise toa deposit of BN.

After each elementary interphase layer has been formed, the gaseousreaction products, including the remainder of the reaction gas, areextracted from the bottom portion of the chamber 12. Extraction isperformed by opening a stop valve 24, thereby putting the chamber 12into communication with a vacuum pump 26 via a liquid nitrogen trap 28that serves to retain undesirable gaseous species and to prevent thembeing exhausted into the environment. Reaction gas extraction byevacuation may be replaced or supplemented by sweeping the chamber 12with an inert gas, such as nitrogen or argon, which gas is injected intothe enclosure from a source 30 via an injection valve 32.

The valves 22a, 22b, 22c, . . . , 24, and 32 are controlled by anautomatic controller 34. The controller also receives signals fromsensors 36 and 38 representing the temperature and the pressure insidethe enclosure. On the basis of these signals, the controller controls anelectrical power supply 16 for the inductor 14 to cause a predeterminedtemperature to exist within the chamber 12, and it controls the stopvalve 24 so that a determined pressure exists within the enclosure priorto each admission of reaction gas.

The lamellar interphase is advantageously made by chemical vaporinfiltration that is implemented in pulsed manner. Each elementary layerconstituting the interphase is formed during a cycle that comprisesinjecting reaction gas corresponding to the nature of the elementarylayer to be formed and maintaining said gas for a predetermined duration(first portion of a cycle), and then extracting the reaction products(second portion of a cycle). Advantageously, cycles follow one anotherwithout interruption.

This succession of cycles is illustrated in FIG. 2. The interphase isformed by an alternation of n elementary layers of the first constituentand of m elementary layers of the second constituent. Each elementarylayer of the first constituent is formed during an A cycle thatcomprises raising the pressure from the value P_(R) of the residualpressure in the chamber to a value P_(A) by admitting a first gas thatgives rise to the first constituent, maintaining pressure for a durationD_(A) for depositing the elementary layer, and then extracting thereaction products until the pressure returns to P_(R). In similarmanner, each elementary layer of the second constituent is formed duringa B cycle comprising raising the pressure from the value P_(R) to avalue P_(B) by admitting a second gas that gives rise to the secondconstituent, maintaining said pressure during a duration D_(B) fordeposition of the elementary layer, and extracting the reaction productsto return to the pressure P_(R).

At the beginning of a cycle, the admission of the reaction gas causesthe pressure in the chamber to rise suddenly. This admission isperformed by the controller 34 causing the corresponding injectionvalve(s) to open for the duration required, given the gas flow rate, toachieve the desired pressure P_(A) (or P_(B)) in the chamber 12. Thisrepresents the partial pressure of the gas constituting the reaction gaswhen it is a single constituent gas, or the sum of the partial pressuresconstituting the reaction gases when a plurality of constituents areinvolved. The pressure P_(A) or P_(B) is selected, as is the temperatureinside the chamber, so as to obtain a deposit of the desired texture andkind. The elementary layer is deposited until the end of the durationD_(A) or D_(B). The stop valve 24 is then opened by the controller 34causing the reaction products to be extracted and causing the pressurein the chamber 12 to drop from the residual value P_(AM) or P_(BM) asachieved at the end of the deposition period down to the value P_(R), atwhich value the pressure is maintained until the beginning of thefollowing cycle.

The duration D_(A) or D_(B) of deposition is selected as a function ofthe thickness desired for the elementary layer. As an indication, forforming a nanometric layer as is required in this case (i.e. a thicknessof less than 10 nanometers) this duration may lie within the range a fewseconds to one minute, it being observed that the deposition ratedepends also on the constituent material of the elementary layer, ontemperature, on pressure, on the shape of the chamber, and on the way inwhich the chamber is filled.

The duration necessary for passing from residual pressure P_(R) topressure P_(A) or P_(B) is generally about one second, and at most a fewseconds, while the duration required for extracting the reactionproducts and for returning to the pressure P_(R) is generally severalseconds, and at most about ten seconds.

As a result, the total duration of a cycle can be limited to a few tensof seconds. Thus, even when several tens of cycles are necessary toachieve the total thickness desired for the interphase (at least 100nanometers and generally several hundreds of nanometers), the total timerequired to form the interphase is relatively short.

The numbers n and m are integers, they may be equal to 1, and they areselected as a function of the desired distribution between the first andsecond constituents in the interphase. These values may be constant orvariable across the thickness of the interphase. It is thus possible tovary the concentration of one of the constituents of the interphaseacross the thickness thereof. In addition, controlled variation in thethickness and/or in the microtexture of the elementary layers of thefirst constituent and/or of the second constituent can be achieved byvarying the parameters D_(A) and/or P_(A) and/or the parameters D_(B)and/or P_(B).

As already mentioned, it is possible to make an interphase having twodifferent constituents while using the same reaction gas. To this end,for one of the cycles, e.g. the B cycle, the deposition duration D_(B)is longer than the deposition duration D_(A) of the A cycle so thatreaction gas depletion gives rise to a change in the nature of thedeposit. The reaction that gives rise to the deposit takes place in aclosed environment, i.e. the reaction gas is not renewed. As a result,the composition and/or the partial pressure thereof can changesufficiently to alter the deposit. This applies when using a gascomprising MTS+H₂ which normally gives rise to a deposit of SiC. It hasbeen observed that when this gas is depleted, and after a certain lengthof time, there arises co-deposition of SiC and of carbon.

In the example shown in FIG. 2, the time interval I_(AA), I_(AB),I_(BB), or I_(BA) between successive different cycles is chosen merelyto ensure that the reaction products are extracted and that the pressurewithin the chamber returns to the residual pressure P_(R). Naturally,that is appropriate when the deposition temperatures are the same for Acycles and for B cycles. Otherwise, each transition I_(AB) or I_(BA)between an A cycle and a B cycle or vice versa would need to be ofsufficient duration for the temperature within the chamber to stabilizeon a value that is suitable for the forthcoming cycle.

Examples of implementations of the invention are described below.

EXAMPLE 1

Monofilaments taken from silicon carbide fiber roving (in fact rovingmade of an Si--C--O composition) sold under the name NICALON NL202 bythe Japanese Nippon Carbon Company were stuck together at their ends ongraphite support frames in order to keep them rectilinear. Each supportframe together with its monofilaments was inserted into an infiltrationchamber. The chamber was filled alternately with propane, and with amixture of MTS/H₂ at a volume ratio of H₂ !/ MTS!=6, under theconditions specified in Table I, so as to coat each of the filaments ina two-constituent interphase comprising PyC and SiC. Infiltration wasperformed using sequences as shown in FIG. 2 by alternating four Acycles having propane admission to form elementary layers of roughlaminar PyC having a thickness of 2.5 nm with six B cycles havingadmission of the MTS/H₂ mixture to form elementary layers of SiC havinga thickness of 1.5 nm. The above was repeated five times to give rise toan overall interphase thickness equal to 2.5×4+1.5×6!×5=95 nm.Thereafter, the monofilaments covered in this way in PyC/SiC interphasewere individually coated in a pure SiC matrix formed by conventionalchemical vapor deposition using an MTS/H₂ mixture so that the volumefraction of fibers in the microcomposites did not exceed about 30%(samples I).

Another series of microcomposites was made by depositing on eachSi--C--O monofilament an interphase having the same overall thickness(100 nm) but constituted solely of PyC, and then coating each coveredmonofilament in an SiC matrix in similar manner (samples II). The PyCinterphase was obtained by chemical vapor infiltration implemented inpulsed manner by performing successive cycles of propane admission suchas the above-described A cycles.

                                      TABLE I                                     __________________________________________________________________________        Nature                                                                              Deposition                                                                         Deposition                                                                         Deposition                                                                          Thickness                                                                           No. of successive                                 of the                                                                              temperture                                                                         pressure                                                                           duration per                                                                        deposited per                                                                       cycles per                                    Deposit                                                                           gas   (K.) (kPa)                                                                              cycle (s)                                                                           cycle (nm)                                                                          sequence                                      __________________________________________________________________________    PyC C.sub.3 H.sub.8                                                                     1273 3    2     ≈2.5                                                                        4                                             SiC MTS + H.sub.2                                                                       1273 3    2     ≈1.5                                                                        6                                                  ##STR1##                                                                 __________________________________________________________________________

A fraction of the microcomposites was tested straight from manufactureand at ambient temperature using a microtraction machine. The resultsare given in Table II and they show that the breaking characteristics ofthe two families of material are similar, with microcomposites having atwo-constituent interphase (PyC/SiC)₅ (samples I) having no significantadvantage over micro-composites having a PyC interphase (samples II)when tested under such conditions.

In Table II:

V_(f) designates the volume fraction occupied by the fibers (percentageof the volume of the composite occupied by fibers)

F_(E) designates load at the elastic limit

F_(R) designates breaking (rupture) load.

                  TABLE II                                                        ______________________________________                                               V.sub.f                                                                              ε.sup.E                                                                      F.sub.E                                                                            σ.sup.E                                                                       E    ε.sup.R                                                                    F.sub.R                                                                            σ.sup.R                  Samples                                                                              (%)    (%)    (N)  (MPa) (GPa)                                                                              (%)  (N)  (MPa)                          ______________________________________                                        Inter- 23     0.084  0.161                                                                              249   335  0.186                                                                              0.272                                                                              500                            phase I                                                                       (PyC/                                                                         SiC).sub.5                                                                    Inter- 31     0.08   0.127                                                                              283   346  0.236                                                                              0.260                                                                              550                            phase II                                                                      PyC                                                                           (100 nm)                                                                      ______________________________________                                    

The remainder of the microcomposites were aged under an oxidizingatmosphere (air) while loaded at 75% of their breaking stress (so as togive rise to multiple cracking of the SiC matrix) at temperatures lyingin the range ambient to 1200° C. After cooling, the microcomposites weretested in traction at ambient temperature as described above. It wasobserved that the residual mechanical characteristics on breaking weredegraded as from aging at 600° C. for microcomposites having a PyC-onlyinterphase, whereas the characteristics were maintained substantiallyeven after aging at 1200° C. for microcomposites having the (PyC/SiC)₅interphase. This example shows the advantage that results from using PyCin the interphase as is made possible by the invention in which theinterphase is built up nanometric layer by nanometric layer with PyC(sensitive to oxidation) alternating in controlled manner with SiC(which protects the carbon with silica which it forms when hot in anoxidizing atmosphere). Even if the nanometric sequencing of the PyC/SiCdoes not of itself give rise to a spectacular improvement in this typeof material as compared with a PyC interphase in terms solely oftransferring load, of suitability for deflecting cracks in the matrix,and/or of relieving residual stresses, it nevertheless gives rise to aspectacular improvement in resistance to oxidation under load.

EXAMPLE 2

Example 1 was reproduced using identical Si--C--O monofilaments andbuilding up a nanometrically-sequenced interphase thereon, whilemodifying deposition conditions as shown in Table III.

                                      TABLE III                                   __________________________________________________________________________        Nature                                                                              Deposition                                                                         Deposition                                                                         Deposition                                                                          Thickness                                                                           No. of successive                                 of the                                                                              temperture                                                                         pressure                                                                           duration per                                                                        deposited per                                                                       cycles per                                    Deposit                                                                           gas   (K.) (kPa)                                                                              cycle (s)                                                                           cycle (nm)                                                                          sequence                                      __________________________________________________________________________    PyC C.sub.3 H.sub.8                                                                     1173 3    15    ≈1                                                                          5                                             SiC MTS + H.sub.2                                                                       1173 1    2     ≈2                                                                          5                                                  ##STR2##                                                                 __________________________________________________________________________

The forces that must be exerted to cause them to break are close tothose obtained in the preceding example (Table II).

These microcomposites and microcomposites having a conventionalpyrocarbon interphase were maintained under traction at 600° C. in airloaded at 70% of their breaking strength.

The microcomposites having a nanometrically-sequenced interphase brokeafter a mean duration of 40 hours whereas those having a pyrocarboninterphase broke after a duration of 20 hours, thereby confirming theadvantage of such lamellar interphases in an oxidizing environment.

EXAMPLE 3

Example 1 was repeated, replacing the Si--C--O-- fiber monofilamentsfrom the Nippon Carbon Company with monofilaments taken fromSi--C--Ti--O fiber roving sold under the name "Tyrano" by the JapaneseUBE Company. The results of traction tests, given in Table IV, show thatthe interphase with nanometric sequencing (PyC/SiC)₅ (samples III) didnot significantly improve the breaking characteristics of themicrocomposites (Young's modulus was greater but stress and deformationat breakage were smaller) as compared with the PyC interphase (samplesIV) when the materials were tested immediately after being made. Incontrast, after aging in air at 800° C. for 24 hours, the residualbreaking characteristics of microcomposites having the (PyC/SiC)₅interphase were conserved whereas those of the PyC interphase becamemuch smaller (σ^(R) <120 MPa).

                  TABLE IV                                                        ______________________________________                                               V.sub.f                                                                              ε.sup.E                                                                      F.sub.E                                                                            σ.sup.E                                                                       E    ε.sup.R                                                                    F.sub.R                                                                            σ.sup.R                  Samples                                                                              (%)    (%)    (N)  (MPa) (GPa)                                                                              (%)  (N)  (MPa)                          ______________________________________                                        Inter- 63     0.12   0.032                                                                              266   241  0.302                                                                              0.065                                                                              600                            phase III                                                                     (PyC/                                                                         SiC).sub.5                                                                    Inter- 76     0.12   0.026                                                                              257   213  0.475                                                                              0.094                                                                              900                            phase IV                                                                      PyC                                                                           (100 nm)                                                                      ______________________________________                                    

EXAMPLE 4

Example 1 was reproduced by creating on Si--C--O monofilaments ananometrically-sequenced interphase, not by modifying the nature of thereaction gases injected into the infiltration chamber, but by using thesame reaction gas while sequentially increasing the duration of one (ormore) deposition periods. As when depositing SiC from the CH₃ SiCl₃ /H₂mixture, this procedure gave rise to in situ depletion of CH₃ SiCl₃ inthe gas and to codeposition of SiC+C. Under such circumstances, theinterphase was no longer constituted by a (PyC/SiC)_(n) sequence, but bya (SIC+PyC)/SiC!_(n) sequence in which the PyC lamellae were replaced bylamellae of codeposited PyC+SiC, the lamellae of pure SiC remainingunchanged. After deposition of the interphase, the SiC matrix wasdeposited as described in Example 1. The deposition conditions for theelementary layers of SiC+C and of SiC are given in Table V.

                  TABLE V                                                         ______________________________________                                                Deposi-  Deposi- Deposi-                                                                              Thick-  No. of                                        tion tem-                                                                              tion    tion dura-                                                                           ness de-                                                                              successive                            Deposit perature pressure                                                                              tion per                                                                             posited per                                                                           cycles per                            composition                                                                           (K)      (kPa)   cycle (s)                                                                            cycle (nm)                                                                            sequence                              ______________________________________                                        Atomic  1273     3       20     ≈3.5                                                                          2                                     50/50                                                                         SiC + C                                                                       SiC     1273     3        2     ≈1.5                                                                          6                                     ______________________________________                                    

The microcomposites of the (SIC+PyC)/SiC!_(n) interphase were tested intraction at ambient temperature. Their mechanical characteristics atbreakage were close to (although slightly poorer than) those given inTable II for corresponding (PyC/SiC)_(n) microcomposites. In contrast,the fact of reducing the overall free carbon content in the interphaseand above all of dispersing the free carbon on a nanometric scale withinthe unoxidizable SiC material had the effect of giving microcompositeswith the (SIC+PyC)/SiC!_(n) interphase better strength under load in aoxidizing medium. This example shows the possibilities provided by theinvention for controlled construction of the interphase in ceramicmatrix composites.

EXAMPLE 5

Fabrics made up of Si--C--O fibers (NICALON fibers from the NipponCarbon Company) were stacked in tooling and then a first batch wascovered (samples V) in a nanometrically-sequenced PyC/SiC interphasehaving an overall thickness of 300 nm with a gradient of SiC compositionrelative to thickness (SiC concentrating going from 10% by volume at thefiber/interphase interface to about 90% by volume in the vicinity of theinterphase/matrix interface), and a second batch (samples VI) werecovered in a 100 nm thick PyC interphase by conventional chemical vaporinfiltration. The two preforms treated in this way were densified by anSiC matrix by conventional chemical vapor infiltration. The SiCcomposition gradient in the interphase was obtained by progressivelyincreasing the ratio m/n across the series of SiC layers alternatingwith series of PyC layers.

Two types of mechanical tests were performed on parallelepipedal testpieces having dimensions of 60 mm×10 mm×3 mm cut out from the resultingmaterials: (i) traction tests were performed at ambient temperature bothbefore and after aging under load in air; and (ii) 4-point bending testswere performed both before and after aging in air (top points 25.4 mmapart, bottom points 50.8 mm apart). Table VI gives the results of thetests.

                  TABLE IV                                                        ______________________________________                                        Traction at ambient  Bending at ambient                                       temperature          temperature                                              Samples σ.sup.R (MPa)                                                                    σ.sup.R * (MPa)                                                                     σ.sup.R (MPa)                                                                  σ.sup.R ** (MPa)                    ______________________________________                                        V       200      180         440    420                                       (300 nm)                                                                      VI      180      <10         420     ≈0                               (100 nm)                                                                      ______________________________________                                         *After 40 hours in air at 1000° C.                                     **After 20 hours in air at 1200° C. under bending.                

Table VI shows that composites V and VI immediately after being madehave similar breaking strengths. In contrast, after being aged in air,the composite having a conventional PyC interphase (samples VI) had lostnearly all of its strength whereas the composite having an interphasethat was sequenced and that had a composition gradient (samples V) inwhich carbon was dispersed in SiC, retained practically all of itsinitial breaking strength.

In addition, the same materials, when subjected to a traction stress of150 MPa (giving rise to multiple cracking in the SiC matrix) at 600° C.in an oxidizing atmosphere (air) had very different lifetimes: materialV did not break after 100 hours whereas material VI broke after 25hours.

Although the above description relates to making interphases having twoconstituents, it is possible to envisage making interphases out of morethan two constituents, e.g. by using a plurality of constituents havinglamellar microtexture and/or a plurality of refractory constituentshaving the function of providing protection against oxidation.

In addition, although the constituent having lamellar microtexture inthe above examples is PyC of the rough laminar type, it is naturallypossible to use some other constituent having a similar microtexture,such as BN or BC₃. BN can be deposited from a precursor comprising amixture of BF₃ +NH₃ (at a volume ratio of 1/2). Both gases are sucked inindependently from cylinders of BF₃ and NH₃ and they are mixed togetheronly after penetrating into the infiltration chamber so as to avoidreaction products forming in the pipework. The infiltration temperatureis about 1050° C. and the maximum pressure reached during a depositioncycle is about 3 kPa.

We claim:
 1. A method of manufacturing a composite material comprisingthe steps of:providing a fiber reinforcement; coating the fibers in saidfiber reinforcement with an interphase layer by carrying out a pluralityof deposition sequences, each sequence including a plurality ofdeposition cycles, each cycle comprising depositing an elementary layerhaving a thickness of less than 10 nanometers, and each sequencecomprising the forming of at least one elementary layer in a firstconstituent which intrinsically presents a lamellar microtexture and atleast another one elementary layer in a second constituent which iscapable of protecting the first constituent against oxidation, wherebythe interphase layer is formed by superposed elementary layers; anddensifying said fiber reinforcement having said interphase coating witha matrix.
 2. A method according to claim 1, characterized in that one ormore elementary layers of a first lamellar microtexture constituent areformed in alternation with one or more elementary layers of a secondconstituent which is capable of providing protection against oxidation.3. A method according to claim 1, characterized in that the firstconstituent comprises pyrolytic carbon, boron nitride, or BC₃.
 4. Amethod according to claim 1, characterized in that the secondconstituent is a refractory material, which has healing properties,either intrinsically or as a result of oxidation, whereby the materialis capable of taking up a semi-liquid state.
 5. A method according toclaim 1, characterized in that the second constituent comprises SiC, Si₃N₄, SiB₄, SiB₆, or a codeposit of the elements Si, B, and C.
 6. A methodaccording to claim 1, characterized in that the interphase is formed bychemical vapor infiltration or deposition within a chamber in which aplurality of successive cycles are performed, each comprising injectinga reaction gas and maintaining it within the chamber for a first timeinterval having a predetermined duration to form an elementaryinterphase layer of controlled thickness of nanometer order, followed byevacuating gaseous reaction products during a second time interval,cycles being performed consecutively in the chamber until the interphasereaches the desired thickness.
 7. A method according to claim 1,characterized in that the interphase is formed by chemical vaporinfiltration or deposition from a single reaction gas resulting in twoelementary layers having a difference, the difference obtained bychanging the duration of the cycle of formation during which theelementary layers are formed.
 8. A method according to claim 1,characterized in that the concentration of one of the constituents ofthe interphase is caused to vary throughout the thickness thereof.
 9. Acomposite material comprising a fiber reinforcement, a matrix densifyingthe fiber reinforcement and an interphase coating layer interposedbetween the fibers of the fiber reinforcement and the matrix,whereinsaid interphase layer includes a plurality of coating sequences, eachsequence including a plurality of elementary layers having a thicknessof less than 10 nanometers and comprising at least one elementary layerin a first constituent which intrinsically presents a lamellarmicrotexture and at least another one elementary layer in a secondconstituent which is capable of protecting the first constituent againstoxidation, whereby the interphase layer is formed by superposedelementary layers.
 10. A material according to claim 9, characterized inthat the interphase is constituted by an alternation of one or moreelementary layers of the first constituent having lamellar microtextureand one or more elementary layers of the second constituent which iscapable of providing protection against oxidation.
 11. A materialaccording to claim 9, characterized in that the first constituentcomprises pyrolytic carbon, boron nitride, or BC₃.
 12. A materialaccording to claim 9, characterized in that the second constituent is arefractory material, presenting healing properties intrinsically or as aproduct of oxidation, whereby the material is capable of taking up asemi-liquid state.
 13. A material according to claim 9, characterized inthat the second constituent comprises SiC, Si₃ N₄, SiB₄, SiB₆, orco-deposition of the elements Si, B, and C.